NEW YORK (CNNMoney) -- The Secret Service is investigating a reported credit card data breach at discount retailer Target.
Secret Service spokesman Brian Leary confirmed the investigation Wednesday evening. He declined to comment further.
Leary's comments came following a report from respected security researcher Brian Krebs that Target had suffered a data breach around the time of Black Friday last month "potentially involving millions of customer credit and debit card records."
Target did not respond to requests for comment Wednesday evening.
Krebs reported that the breach apparently targeted customers at stores rather than online shoppers. The thieves reportedly gained access to data on the magnetic strips of shoppers' cards, potentially allowing them to produce counterfeit versions.
The thieves could also potentially withdraw cash from ATMs using counterfeit debit cards if they were able to intercept PIN data from Target, Krebs said.
American Express and Discover both said they were "aware" of the incident and had fraud controls in place.
"This is an ongoing investigation," an AmEx spokeswoman said, declining to comment further.
MasterCard referred questions to Target; Visa did not respond to requests for comment.
Target competitor TJX Companies -- which operates discount retail chains T.J. Maxx and Marshalls -- fell victim to one of the worst security breaches ever back in 2006, when hackers gained access to at least 94 million domestic and international accounts containing credit card, debit card, and check information.
Target released the following statement:
We wanted to make you aware of unauthorized access to Target payment card data. The unauthorized access may impact guests who made credit or debit card purchases in our U.S. stores from Nov. 27 to Dec. 15, 2013. Your trust is a top priority for Target, and we deeply regret the inconvenience this may cause. The privacy and protection of our guests’ information is a matter we take very seriously and we have worked swiftly to resolve the incident.
We began investigating the incident as soon as we learned of it. We have determined that the information involved in this incident included customer name, credit or debit card number, and the card’s expiration date and CVV (the three-digit security code).
We are partnering with a leading third-party forensics firm to conduct a thorough investigation of the incident and to examine additional measures we can take that would be designed to help prevent incidents of this kind in the future. Additionally, Target alerted authorities and financial institutions immediately after we discovered and confirmed the unauthorized access, and we are putting our full resources behind these efforts.
We recommend that you closely review the information provided in this letter for some steps that you may take to protect yourself against potential misuse of your credit and debit information. You should remain vigilant for incidents of fraud and identity theft by regularly reviewing your account statements and monitoring free credit reports. If you discover any suspicious or unusual activity on your accounts or suspect fraud, be sure to report it immediately to your financial institutions. In addition, you may contact the Federal Trade Commission (“FTC”) or law enforcement to report incidents of identity theft or to learn about steps you can take to protect yourself from identity theft. To learn more, you can go to the FTC’s Web site, at www.consumer.gov/idtheft, or call the FTC, at (877) IDTHEFT (438-4338) or write to Federal Trade Commission, Consumer Response Center, 600 Pennsylvania Avenue, NW, Washington, DC 20580.
You may also periodically obtain credit reports from each nationwide credit reporting agency. If you discover information on your credit report arising from a fraudulent transaction, you should request that the credit reporting agency delete that information from your credit report file. In addition, under federal law, you are entitled to one free copy of your credit report every 12 months from each of the three nationwide credit reporting agencies. You may obtain a free copy of your credit report by going to www.AnnualCreditReport.com or by calling 877-322-8228. You may contact the nationwide credit.
Again, we want to stress that we regret any inconvenience or concern this incident may cause you. Be assured that we place a top priority on protecting the security of our guests’ of personal information. Please do not hesitate to contact us at 866-852-8680 or visit Target’s website if you have any questions or concerns.